The present invention relates to a method of fabricating a thin-film transistor using a crystalline silicon film.
Thin-film transistors (TFTs) having an active layer made of a thin film of silicon are known. These TFTs are chiefly put into practical use in active-matrix liquid crystal displays.
TFTs currently made practicable are classified into amorphous silicon TFTs (a-Si TFTs) using amorphous silicon film and high-temperature p-Si TFTs fabricated using ordinary IC technology.
High-temperature p-Si is obtained by making use of a technology for obtaining a crystalline silicon film by utilizing a high-temperature treatment performed over 900xc2x0 C. Where high characteristics are required, it is desired to use a crystalline silicon film. At the heat treatment temperature necessary for manufacturing high-temperature p-Si film, however, it is impossible to use a glass substrate.
TFTs are mainly used in LCDs and demand the use of glass substrates. In an attempt to satisfy this requirement, research has been conducted into a technique for fabricating a crystalline silicon film by process steps performed at low temperatures that glass substrates can withstand. These processes are known as low-temperature processes in contrast to high-temperature processes for fabricating high-temperature p-Si. Crystalline silicon film fabricated by such low-temperature processes are known as low-temperature p-Si, and TFTs using low-temperature p-Si film are termed low-temperature p-Si TFTs.
Techniques for fabricating low-temperature p-Si films can be roughly classified into a method using laser irradiation and a method using heating. The method using laser irradiation is characterized in that it hardly thermally damages a glass substrate because the laser light is directly absorbed near the surface of an amorphous silicon film. However, the stability of the used laser presents problems. In addition, this method cannot cope with large-area films satisfactorily.
On the other hand, with respect to the method using heating, the required crystalline silicon film cannot be obtained by a heat treatment performed at a temperature to which a glass substrate can stand up.
We have proposed a technique for alleviating the existing problems described above in Japanese Unexamined Patent Publication No. 268212/1994. In this technique, a metallic element for promoting crystallization of silicon, typified by nickel, is maintained in contact with the surface of an amorphous silicon film. Then, a heat treatment is performed. Thus, a crystalline silicon film having the requisite crystallinity can be formed at a temperature which is lower than the temperature used heretofore and to which the glass substrate and stand up. This crystallization technique using nickel is useful in that a crystalline silicon film having the required crystallinity can be obtained by a heat treatment conducted at a temperature low enough that the glass substrate can withstand.
However, the nickel used for the crystallization inevitably remains in the active layer. This leads to instability of the characteristics of the finished TFTs and to deterioration of the reliability.
It is an object of the present invention to provide a method of forming a crystalline silicon film, using the aforementioned metallic element for promoting crystallization of silicon, in such a way as to eliminate the effects of nickel remaining in the obtained silicon film.
One embodiment of the present invention is illustrated in a process sequence of FIG. 1 and comprises the steps of: forming a silicon film 104 crystallized by the action of a metallic element (such as nickel) for promoting crystallization of silicon; forming a mask 105 for exposing parts of the silicon film; forming a film 106 containing a XV group (group 15) element (such as phosphorus) to cover parts of the exposed silicon film 104 and the mask 105; and performing a heat treatment (FIG. 1(D)) to move the metallic element from the silicon film 104 to the film 106 containing the XV group (group 15) element.
In the structure described above, as for the nickel moved by the heat treatment, the silicon films 104 and 106 are joined together. That is, the nickel moved by the heat treatment cannot distinguish between the silicon films 104 and 106.
Accordingly, in the heat treatment step shown in FIG. 1(D), the nickel element contained in the silicon film 104 diffuses into the silicon film 106. Note that the metallic element hardly diffuses into the mask 105 consisting of a silicon oxide film.
On the other hand, the silicon film 106 is heavily doped with phosphorus and serves as gettering sites for nickel. Therefore, the nickel moved into the silicon film 106 bonds with phosphorus and becomes stable.
If the heat treatment temperature used in the step of FIG. 1(D) is set lower than 800xc2x0 C., preferably lower than 750xc2x0 C., phosphorus hardly diffuses through the silicon film and so nickel once accepted into the silicon film 106 stops there. The nickel does not diffuse back into the silicon film 104.
In this way, the nickel in the silicon film 104 moves into the silicon film 106. It can be said that the nickel in the silicon film 104 is gettered and driven into the silicon film 106.
During the heat treatment shown in FIG. 1(D), the whole silicon film 106 acts as gettering sites. Therefore, even if the area of the portion of the silicon film 104 in contact with the silicon film 106 is relatively small, movement of the nickel can be made effectively. That is, the concentration of nickel in the silicon film 104 can be effectively reduced.
Another embodiment of the invention is illustrated in FIGS. 3(A)-3(E) and comprises the steps of: forming a mask 302 on an amorphous silicon film 301 and exposing parts of the mask by openings 303 formed in the mask; selectively introducing a metallic element for promoting crystallization of silicon into the exposed regions of the amorphous silicon film (FIG. 3(B)); performing a heat treatment to diffuse the metallic element from the exposed regions into the silicon film (FIG. 3(C)); forming a silicon film 307 containing phosphorus on the mask 302 and bringing the phosphorus-containing silicon film into contact with the metal-diffused silicon film in the exposed regions (FIG. 3(D)); and performing a heat treatment to move the metallic element from the metal-diffused silicon film into the phosphorus-doped film via the exposed regions as indicated by the arrow 308 (FIG. 3(D)).
Where the method described above is adopted, nickel can be removed from the region in which the metallic element has been introduced (the region having the openings 303), by making use of the mask 302 used to induce crystal growth, known as lateral growth, as shown in FIG. 3(C). The same mask pattern can be used both for the introduction of the metallic element and for the removal of the metallic element. Consequently, the process sequence is not complicated so much.
During the nickel-removing step shown in FIG. 3(D), the silicon film 307 is far larger in area than the openings 303. When the nickel is diffusing into the silicon film 307, the nickel is effectively gettered out of the openings 303 in the mask and driven into the silicon film 307.
Nickel is the most preferred metallic element for promoting crystallization of silicon. Use of phosphorus (P) is more preferable as the XV group (group 15) element. Where a combination of nickel and phosphorus is used, the present invention yields the best advantages.
One or more elements selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Au, Ge, Pb, and In can be employed as the metallic element for accelerating crystallization of silicon. Any one element P, As, or Sb can be selected as the XV group (group 15) element.
Other objects and features of the invention will appear in the course of the description thereof, which follows.